It is known that aqueous systems, particularly industrial aqueous systems such as boiler systems, cooling tower systems, heat exchanger systems, desalinization systems, and the like, are subject to the formation of deposits on the internal surfaces which are in contact with the circulating water. Depending upon the nature of the deposit, various approaches have been taken to either control or inhibit the formation of these deposits or alternatively to remove the already formed deposits. For example chelants are known for their ability to complex with many cations including hardness and heavy metal cations and thereby prevent their deposition. Chelants function by essentially "locking" the metal into a soluble organic ring structure. When chelants are applied in combination with a dispersant, the treated system often results in clean waterside surfaces. However, chelants tend to be somewhat specific with respect to which cations they will complex with. Anionic chelants have reactive sites that attract coordination cites of cations; coordination sites are areas on the ion that are receptive to chemical bonding. Thus for example, iron has six coordination sites, as does the chelant EDTA. Thus, iron ions combine with the EDTA and since all of the coordination sites on the iron ion are chemically bonded by the EDTA, a stable metal chelant is formed. However, nitrilotriacetic acid (NTA), which is another commonly used chelant, has only four coordination sites, and thus NTA does not form as stable a metal chelant complex as does EDTA. Citric acid is a moderately strong Ca(II) chelant, but a weak ferric chelant.
U.S. Pat. No. 4,721,532 discloses certain alkylene amine carboxyl polyacids (AACPs) which are useful for removing iron fouling from heat transfer surfaces of cooling water systems. Particularly preferred compounds disclosed therein are N,N'-ethylene-bis-((2-hydroxy-4-methylphenyl)glycine) (EDDHMA) and N,N-di-(2-hydroxy-5-sulfonic acid benzyl) glycine, (Hamplex DPS). However, while these compounds have been found to be useful for removing iron fouling, they are less effective in removing rust scale containing both iron and calcium.
It has now been discovered that 3,5-bis(di-N,N- (carboxymethyl)aminomethyl)-4-hydroxybenzenesulfonic acid provides surprising effectiveness in chelating not only iron but also effectively chelating calcium ions in aqueous solutions and is thus effective at preventing and removing both calcium and rust scale deposits.